1. Field of the Invention
The present invention relates generally to the detection of defects in selectively permeable barriers through which electrochemical sensors communicate with a sample material to be analyzed and, more particularly, to a method for checking the integrity of such a barrier associated with a sensor incorporated in an electrochemical titrator.
2. Description of the Prior Art
Copending U.S. Pat. application Ser. No. 586,435 (now U.S. Pat. No. 4,003,705), by Buzza et al., and assigned to the assignee of the present invention, describes an electrochemical analysis apparatus for measuring both chloride and carbon dioxide in blood. The blood sample is reacted with an acid reagent in a sample chamber to release carbon dioxide which diffuses through a gas permeable membrane to a carbon dioxide sensor. The sensor includes an alkaline reagent which undergoes a change in pH upon reaction with the carbon dioxide and a pH measuring electrode arrangement for measuring the resulting pH change to provide a measure of the carbon dioxide concentration. Specifically, the pH signal is differentiated to provide an instantaneous time rate of change of pH signal which is measured to determine the carbon dioxide value.
For chloride detection, the aforementioned apparatus includes titration apparatus comprising coulometric generator electrodes and amperometric detector electrodes communicating with the sample chamber for titrating the sample chloride. To this end the coulometric generator electrodes are energized to generate silver ions which combine with the sample chloride to precipitate silver chloride. The completion of the silver chloride precipitation is detected by the amperometric detector electrodes and the total coulometric current flow required to precipitate the silver chloride provides a measure of the chloride originally in the sample. To ensure that the measured coulometric current represents titration of the sample chloride only, the coulometric generator is energized prior to sample introduction to precipitate out chloride which may be present in the acid reagent to establish an initial or base line chloride level in the sample chamber prior to the introduction of each sample.
It is critically important that the selectively permeable membrane permit the passage of carbon dioxide to the carbon dioxide sensor but reject all other substances which could adversely affect operation of the carbon dioxide sensor. Moreover, the membrane must provide electrical isolation between the carbon dioxide sensor and the titration circuitry. If the membrane includes pin holes or other defects which permit reagent leakage through or around the membrane to the carbon dioxide sensor or which permit electrical cross-talk between the carbon dioxide measuring and chloride measuring systems, then erroneous sample measurements will result. Typically, for adequate isolation a membrane should exhibit an impedance of about 10.sup.9 ohms or more.
Several proposals have been advanced in the past for monitoring the isolation integrity (i.e. impedance) of selectively permeable membranes in electrochemical sensor arrangements. In one approach an a.c. signal is superimposed across the membrane and a.c. detector and demodulation circuitry is provided to monitor the a.c. signal transmitted by the membrane. In another approach a separate monitoring electrochemical half cell and associated monitoring circuitry is provided for developing a signal across the membrane indicative of a fault in the membrane. While each of these approaches is generally satisfactory for the purpose intended, each requires the addition of independent and complex electrical monitoring circuitry and components which reduces the overall attractiveness of each for commercial implementation particularly for combined electrochemical sensing and titration apparatus as described above. As a result, a need exists for a simple and inexpensive technique for checking the integrity of a selectively permeable barrier in such apparatus.